This invention relates to electrostatic printing and, more particularly, to an improved electrostatic printing master adapted for the use of conventional silver halide photographic techniques during preparation of the master for printing.
Electrostatic printing is well-known in the art and has been proposed as an alternative to other printing techniques. In one method of electrostatic printing, one first prepares a "master" that is capable of selectively holding electrostatic charges to form the desired image. The master is exposed to a corona discharge that forms a latent electrostatic image, and contacted with dry or liquid toner of the opposite electrostatic charge to develop the image. The toned image is then transferred to a substrate, typically paper, where the toner is fused to fix the image, and the master is returned for the next printing cycle.
It has been suggested in U.S. Pat. No. 4,069,759 that an improved electrostatic printing master can be fabricated by dispersing a conventional silver halide photographic salt in an insulating polymer (e.g., gelatin), and coating the dispersion on a conducting substrate. The coating is exposed imagewise, and is developed to cause the exposed silver halide to be reduced to metallic silver. The unexposed silver halide is then dissolved and removed from the coating to fix the image. While the master suggested in U.S. Pat. No. 4,069,759 offers many advantages; and permits the use of conventional aqueous silver halide photographic chemistry when gelatin is selected as the insulating polymer, it has been found that gelatin is too highly sensitive to humidity to have practical application in a typical workplace. Gelatin rapidly absorbs moisture from the air and at moderate to high humidities no longer functions as an insulating medium, but provides a conductive path that grounds surface charges imposed on the master during the electrostatic printing process.
Thus, there is a need for an improved electrostatic printing master that will offer the advantages of being based on conventional aqueous silver halide photographic chemistry and provide superior insulating properties under relative humidity conditions commonly encountered during printing.